The plasma etching process employs an apparatus as disclosed in U.S. Pat. No. 4,534,816, Lee Chen, "Single Wafer Plasma Etch Reactor" (issued Aug. 13, 1985), U.S. Pat. No. 4,780,169, Mark M., "Non-uniform Gas inlet for Dry Etching Apparatus" (issued Oct. 25, 1988), U.S. Pat. Nos. 5,423,936, 5,445,709, and others.
The disclosed apparatus is provided with paired parallel plate electrodes facing each other. One of them is located at the lower part of the chamber so that wafers (substrates) are placed on it. Another of them is located at the upper part of the chamber and has a number of small through holes for gas introduction. For etching, this apparatus applies a high-frequency electric power across the electrodes to induce discharge while introducing a reactive gas (such as a halogen gas and freon gas) through the upper electrode. The resulting gas plasma attacks that part of the substrate which is not protected by the photoresist. In this way accurate fine circuit patterns are formed on the semiconductor wafer.
The upper electrode for plasma etching is a disk having a number of small through holes as shown in Lee Chen's patent (FIG. 2) or Mark's patent (FIG. 3A). It varies in shape, number, and arrangement of small holes depending on the apparatus and etching conditions. (In other words, not all electrodes of the same type can be used in common for any apparatus.) The shape of small holes strictly depends on the kind of electrode for individual apparatus.
Additional requirements for the electrode plate include good electrical conductivity, high purity (to prevent wafer contamination), and chemical stability (to protect itself from etching). To meet these requirements, early electrodes were made of aluminum, stainless steel, or carbon (graphite). Electrode plates made of glassy carbon have come into general use recently.
Glassy carbon is a hard, macroscopically non-porous carbonaceous substance obtained by carbonization of a thermosetting resin. It is characterized by a high strength, chemical inertness, gas impermeability, self-lubrication, toughness, and purity. Another advantage over other materials is that it has the least likely possibility of fine particles releasing themselves from the structure to contaminate the wafer during plasma etching.
The ever-increasing degree of integration needs plasma-etching electrodes to meet requirements for configuration as well as physical and chemical properties. In other words, the plasma-etching electrodes should be at least liable to wafer contamination with particles and to wearing. Efforts have been made to improve the plasma-etching electrodes of glassy carbon.
Some examples of improvements are as follows.
(1) A glassy carbon material for a plasma apparatus which is characterized by having a porosity of 0.0002-0.0020%, a crystallite undetectable by X-ray diffraction, and an impurity content lower than 5 ppm. In other words, it has improved purity, porosity, pore diameter, and crystalline structure. (Japanese Patent Laid-open No. 33007/1991) PA1 (2) A plasma-etching electrode plate of high-purity glassy carbon having the structure characterized by having a pore diameter smaller than 1 .mu.m (maximum) or 0.7 .mu.m (average) and a porosity lower than 1%. (EP421668B1) PA1 (3) A plasma-etching electrode plate (thicker than 2 mm) of high-purity glassy carbon which is substantially free of grain boundaries in the surface and inner structure and has a pore diameter smaller than 1 .mu.m (maximum). (Japanese Patent Laid-open No. 285086/1991) PA1 (4) A plasma-etching electrode plate of glassy carbon characterized by having an ash content lower than 5 ppm, metal impurities lower than 2 ppm, a total sulfur content lower than 30 ppm, a specific gravity higher than 1.50, a flexural strength higher than 1100 kg/cm.sup.2, and a crystalline structure with a crystal interlayer distance (002) smaller than 0.375 nm and a crystallite size (002) larger than 13 nm. (Japanese Patent Laid-open No. 320955/1993) PA1 (5) A plasma-etching electrode plate of glassy carbon composed of crystals having a lattice constant smaller than 6.990 .ANG.. (Japanese Patent Laid-open No. 128761/1994) PA1 (6) A plasma-etching electrode plate of glassy carbon having a surface roughness (R.sub.max) lower than 6 .mu.m at that part of the surface which is subject to wear by plasma. (Japanese Patent Laid-open No. 128762/1994) PA1 (7) A plasma-etching electrode plate of glassy carbon made of a phenolic resin and a polycarbodiimide resin. (Japanese Patent Laid-open No. 347270/1993) PA1 (8) A plasma-etching electrode plate of glassy carbon made of a polycarbodiimide resin. (Japanese Patent Laid-open No. 347278/1993) PA1 (9) A thick glassy carbon plate made by the process disclosed in Japanese Patent Laid-open Nos. 362062/1992 and 100365/1994.
One of the important factors to be considered in plasma etching is the rate of etching. For accurate fabrication, it is necessary to maintain a uniform etch rate. Achieving this objective depends on the temperature of the reaction chamber and especially on the temperature distribution on the electrode surface. Any variation in the surface temperature prevents uniform, stable etching.
A common way to keep uniform the surface temperature of a thin electrode plate (which becomes hot due to plasma irradiation) is to tightly attach a cooling metal plate to the back of the electrode plate. A problem in this case is with the warp of the electrode plate. The thinner the electrode plate, the more liable it is to warp. Warp forms a small gap which prevents close contact between the electrode plate and the cooling metal plate, resulting in insufficient heat dissipation. Therefore, it is necessary to devise a method for minimizing warp.
The electrode plate should preferably be thick rather than thin from the standpoint of durability. Good durability prolongs the interval between shutdowns for electrode replacement.
The uniformity of etch rate is important for accurate etching in view of the fact that finer circuit patterns are necessary for semiconductor de vices with a higher degree of integration and the wafer size has exceeded 8 inches and is expected to exceed 12 inches in the near future. Nevertheless, no efforts have been made to improve the uniformity of etch rate from the material viewpoint of the glassy carbon constituting the electrode.
The etch rate is governed mainly by the temperature of the reaction system. In the case of a thick electrode plate, it is important for the uniform etch rate to keep uniform the temperature distribution on the electrode surface. The thick electrode plate of glassy carbon proposed in Japanese Patent Laid-open Nos. 362062/1992 and 100365/1994 mentioned above has be en found unsatisfactory for plasma etching.